This cargo helicopter with tandem rotors was designed for the U.S. Army during the Fifties. It can be considered to be the first real reliable helicopter of its category notwithstanding the limits of its piston engines. The H-21 was also purchased by the French Navy which used it during the Algerian war for transport and attack missions. At the same time the West German and Canadian Armies acquired the choppers for supply and support roles. The U.S. Army used the H-21 in substantial numbers during the initial phase of the Vietnam war.